The beneficial effects of Si are mainly associated with its high deposition in plant tissues, enhancing their strength and rigidity. However, Si might play an active role in enhancing host resistance to plant diseases by stimulating defense reaction mechanisms.

Because many plants are not able to accumulate Si at high enough levels to be beneficial, genetically manipulating the Si uptake capacity of the root might help plants to accumulate more Si and, hence, improve their ability to overcome biotic and abiotic stresses.

A brief history of silicon in plants

Silicon (Si) is the second most abundant element after oxygen in soil. Silicon dioxide comprises 50–70% of the soil mass. However, the role of Si in plant growth and development was overlooked until the beginning of the 20th century. Because of the abundance of the element in nature and because visible symptoms of either Si deficiency or toxicity are not apparent, plant physiologists largely ignored it. However, repeated cropping and the constant application of chemical fertilizers such as nitrogen, phosphorus and potassium have depleted the amount of Si that is available to plants in the soil. An awareness of Si deficiency in soil is now recognized as being a limiting factor for crop production, particularly in soils that are deemed to be low or limiting in plant available Si and for known Si-accumulating plants such as rice and sugarcane. Today, Si still is not recognized as an essential element for plant growth but the beneficial effects of this element on the growth, development, yield and disease resistance have been observed in a wide variety of plant species. Si fertilizers are routinely applied to several crops including rice and sugarcane to enhance high and sustainable crop yields.

Silicon in Plants

Silicon is considered a plant nutrient anomaly because it is presumably not essential for plant growth and development. However, soluble silicon has enhanced the growth and development of several plant species including rice, sugar cane, most other cereals, and several dicotyledons such as cucumber and watermelon. Higher plants vary in their capacity to accumulate silicon. Wetland gramineae (rice) absorb silicon as monosilicic acid, Si(OH)4, equivalent to 4.6% to 6.9% of the dry matter of the rice. Silicon accumulation has been reported to range from 0.5 to 1.5% in dry land gramineae (sugar cane, cereals) and less than 0.2% in dicotyledons (that is, broadleaf plants). Silicon amendments also have proved effective in controlling both soil borne and foliar fungal diseases in cucumber, rice, sugarcane, turf and several other plant species.

Role of Silicon in Stress Resistance

The beneficial effect of Si is more evident under stress conditions. This is because Si is able to protect plants from multiple abiotic and biotic stresses. Numerous studies have shown that Si is effective in controlling diseases caused by both fungi and bacteria in different plant species. For example, Si increases rice resistance to leaf and neck blast, sheath blight, brown spot, leaf scald and stem rot. Silicon also decreases the incidence of powdery mildew in cucumber, barley and wheat; ring spot in sugarcane; rust in cowpea. Two mechanisms for Si-enhanced resistance to diseases have been proposed. One is that Si acts as a physical barrier. Si is deposited beneath the cuticle to form a cuticle–Si double layer. This layer can mechanically impede penetration by fungi and, thereby, disrupt the infection process. Another mechanism proposed recently is that soluble Si acts as a modulator of host resistance to pathogens. Silicon also enhances plant resistance to insect pests such as stem borer and plant hopper. This Si-enhanced effect is attributed to Si deposition in the plant tissues, which provides a mechanical barrier against probing and chewing by insects.

Silicon also alleviates many abiotic stresses including chemical stress (salt, metal toxicity, nutrient imbalance) and physical stress (lodging, drought, radiation, high temperature, freezing) and many others. Most of these beneficial effects are also attributed to Si deposition in cell walls of roots, leaves, stems and hulls. This is particularly important in dense plant stands and when nitrogen fertilizers are heavily applied so as to minimize mutual shading. It has been reported that Si promotes cell elongation but not cell division, probably as a result of Si-enhanced extensibility of the cell wall in rice.

Plants that accumulate large quantities of Si benefit the most because this element enhances stress resistance. If plants are to benefit from Si they must be able to acquire the element in high concentrations regardless of whether they are monocots or dicots. This is particularly important for plants accumulating high levels of Si such as rice and sugarcane. For example, a high level of Si accumulation is required for growth and for the production of a high and sustainable yield of rice. Low Si accumulation results in a significant reduction in rice yield and quality.

Future Prospects

Silicon accumulation in plants is controlled by the ability of roots to take up Si. Silicon uptake is probably a complicated process and might be controlled by multiple genes in rice. Furthermore, given that most plants, particularly dicots, cannot accumulate Si in large enough amounts to be beneficial, genetically manipulating the Si uptake capacity of the roots might help plants to accumulate more Si and, hence, more able to overcome both biotic and abiotic stresses.